The invention relates to a device and to a method for working with jets of cryogenic fluid, in particular liquid nitrogen, at high pressure, particularly for the surface treatment, stripping or scalping of coated or uncoated materials such as metals, concrete, wood, polymers, ceramics and plastics or any other type of material.
At the present time, the surface treatment of coated or uncoated materials, particularly the stripping, scalping or the like, is essentially done by sandblasting, by ultra high pressure (UHP) water spray, using a scourer, a pick hammer, a scabbier or alternatively via a chemical route.
However, when there must not be any water, for example in a nuclear environment, or any chemical product, for example because of severe environmental constraints, only so-called “dry” working processes can be used.
However, in certain instances, these “dry” processes are difficult to implement, are very laborious or are awkward to use or even generate additional pollution, for example because of the addition of shot or sand that has then to be reprocessed.
One alternative to these technologies relies on the use of cryogenic jets at very high pressure, as proposed by documents U.S. Pat. No. 7,310,955 and U.S. Pat. No. 7,316,363. In this case, use is made of one or more jets of liquid nitrogen at a pressure of 1000 to 4000 bar and at a cryogenic temperature of between, for example, −100 and −200° C., typically around −140 and −160° C., which are dispensed by a nozzle-holding tool driven in a rotary movement.
More specifically, this nozzle-holding tool is fixed to the end of a cryogenic fluid conveying pipeline which supplies the tool with cryogenic fluid. The pipeline and the tool are then given a rotary movement about the axis of the pipeline by a drive system involving pinions or belts powered by a motor.
The dynamic sealing of the rotary system is usually afforded by a rotary cylindrical sealing joint, typically made of Tivar®, arranged around the pipeline. Typically, this sealing joint of cylindrical shape has a bronze component passing longitudinally through it and is surrounded with a solid stainless steel component.
Because of the cryogenic temperature involved, it has been found in practice that the effectiveness of this sealing joint decreases as time goes on, and this in the fairly short term leads to leaks and therefore loss of process efficiency, particularly during operations of scalping concrete or stripping paint for example.
Specifically, under the effect of the cryogenic temperatures involved, the materials deform in different ways from one another, according to their respective thermal expansion coefficient, as illustrated in table I.
TABLE IThermal expansion coefficient (×10−6/K)Tivar ®Stainless steelBronze1801517.5
As may be seen, these materials react very differently to the cryogenic temperatures and as a result, during the alternating cooling and heating cycles, deformations or even damage to the sealing joint occur, and this happens all the more rapidly when the sealing joint is subjected to very high pressures, namely typically of up to 4000 bar.
Specifically, it has been found in practice that a clearance progressively appears between the sealing joint and the metal components and gives rise to leakages which prevent normal operation of the system. As a result of this, it is necessary regularly to change the sealing joint, leading to material and maintenance costs. Now, this is of critical importance in hazardous environments, notably in the nuclear or chemical sectors for example, where human intervention is to kept as infrequent as possible.
Document U.S. Pat. No. 4,369,850 describes a device fitted with a nozzle for dispensing water under pressure which nozzle is arranged at the downstream end of a water pipeline, itself arranged in a rotary cylindrical housing rotationally driven by a motor via a belt and pulley transmission mechanism, in which device the water pipeline is flexible and elbowed so as to be able to dispense a jet of water in a circular path so that holes can be made in the ground, that is to say in earth or the like.
However, that device is not entirely satisfactory because it does not allow the surface area impacted by the jet, at a given distance from the nozzle, to be varied, and this proves to be an appreciable disadvantage in certain applications, notably when stripping or scalping the surface, notably concrete.
A similar device is described elsewhere in DE-A-10236266.
In the light of that, the problem addressed is that of proposing a device for dispensing cryogenic fluid, particularly liquid nitrogen, which is reliable, which means to say with which not only do the problems associated with the wearing of the sealing joint and with leakage not exist, so as to remedy the aforementioned disadvantages but which also allows the area treated by the jet or jets of nitrogen at a given distance from the nozzle to be varied, notably when it is being used for stripping or scalping concrete.